HYDROGEN BONDING

HYDROGEN BONDING

 

Hydrogen bonding is an attractive force which occurs in any compound whose molecule contains O-H or O-N bonds (as in water, alcohol, acids, amines, and amides).

The hydrogen is attached directly to one of the most electronegative elements, causing the hydrogen to acquire a significant amount of positive charge.

 Each of the elements to which the hydrogen is attached is not only significantly negative, but also has at least one "active" lone pair. e.g.

Adjacent molecule of the compound containing an O-H bond will be attracted to each other by virtue of these opposite charges. This force of attraction is known as the Hydrogen bonding. Usually hydrogen bond is represented by a dotted line, as shown below.

 

   

Hydrogen bonding has important effect on many physical properties (physical state, molecular mass, boiling points, solubility, acidity etc) of organic compounds.

1.   INCREASE IN THE SIZE AS WELL AS IN THE MOLECULAR MASS:

The weak electrostatic interactions of intermolecular hydrogen bonding, causes two or more molecules of a compound to exist as aggregates or associated molecules. This is seen in water molecules, which undergo molecular association. Alcohols and carboxylic acids (RCOOH) also exist as a group of molecules. Molecular association results in the increase in the size as well as in the molecular mass of the compound. These above examples are shown below:

 

                                                                 (Molecular association of carboxylic acid.)

2. HIGHER MELTING AND BOILING POINTS:

The compounds containing hydrogen bonds have high melting and boiling points. Due to hydrogen bonding, the intermolecular force of attraction in the compound becomes large. Consequently, larger energy is required in separating these molecules before they can melt or boil.

e.g. water molecule, molecular weight= 18 g/mole   B.Point= 100^oC,

while the methane with comparable weight is a gas.

It is important to realise that hydrogen bonding exists in addition to van der Waals attractions. For example, all the following molecules contain the same number of electrons, and the first two are much the same length. The higher boiling point of the butan-1-ol is due to the additional hydrogen bonding.

Comparing the two alcohols (containing -OH groups), both boiling points are high because of the additional hydrogen bonding due to the hydrogen attached directly to the oxygen - but they aren't the same.

The boiling point of the 2-methylpropan-1-ol isn't as high as the butan-1-ol because the branching in the molecule makes the van der Waals attractions less effective than in the longer butan-1-ol.

3. INFLUENCE ON THE PHYSICAL STATE:

Hydrogen bonding also influences the physical state of the substances (solid, liquid or gas).

Illustration:

Both oxygen and Sulphur belong to same group but H₂O is a liquid at ordinary temperature, while H₂S is a gas. This is explained on the basis of electronegativity values.

In water, oxygen is highly electronegative so that it forms hydrogen bonds. As a result the molecules of H₂O get associated with one another and this raises the boiling point of water. Consequently, water exists as liquid at room temperature. On the other hand, the difference in electronegativity of atoms in H₂S is less and hydrogen bonding in H₂S is almost negligible. As a result H₂S is not associated and exists as a gas at room temperature.

4. SOLUBILITY:

Hydrogen bonding also influences the solubility of one substance in another. Covalent compounds do not generally dissolve in water, but those that can form a hydrogen bond with water, readily dissolve in it. For example, alcohols like ethanol, ammonia, amines, lower aldehydes and ketones are soluble in water due to the formation of hydrogen bonds with water molecules.

                                             While

 

Hydrogen bonding also occurs in organic molecules containing N-H groups - in the same sort of way that it occurs in ammonia. Examples range from simple molecules like CH₃NH₂ (methylamine) to large molecules like proteins and DNA.

The two strands of the famous double helix in DNA are held together by hydrogen bonds between hydrogen atoms attached to nitrogen on one strand, and lone pairs on another nitrogen or an oxygen on the other one.